Methods of producing bipolar transistors having emitter-base junctions of varying depths and/or doping concentrations

ABSTRACT

Methods for producing bipolar transistors are provided. In one embodiment, the method includes producing a bipolar transistor including first and second connected emitter-base (EB) junctions of varying different depths. A buried layer (BL) collector is further produced to have a third depth greater than the depths of the EB junctions. The emitters and bases corresponding to the different EB junctions are provided during a chain implant. An isolation region may overlie the second EB junction location. The BL collector is laterally spaced from the first EB junction by a variable amount to facilitate adjustment of the transistor properties. The BL collector may or may not underlie at least a portion of the second EB junction. Regions of opposite conductivity type overlie and underlie the BL collector to preserve breakdown voltage. The transistor can be readily “tuned” by mask adjustments alone to meet various device requirements.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of co-pending U.S. application Ser. No.12/909,632, filed Oct. 21, 2010.

FIELD OF THE INVENTION

The present invention generally relates to semiconductor devices andcircuits and methods for fabricating bipolar transistors havingemitter-base junctions of varying depths, doping concentrations, orboth.

BACKGROUND OF THE INVENTION

Bipolar transistors are much used in modern electronics as individualdevices and as part of various integrated circuits (ICs). It is oftennecessary to adjust (e.g., to “tune”) the properties of particularbipolar transistors to suit specific applications, for example and notintended to be limiting, to adjust one or more of the gain, Earlyvoltage, breakdown voltages, current carrying capacity, high-sidecapability, etc. It is sometimes the case that improving one property(e.g., gain), can adversely affect other properties (e.g., Early voltageand/or breakdown voltage, etc.). This is undesirable. In the prior artit has been customary to tune the properties of particular devices by,for example, adjusting the doping profiles. However, in large scaleproduction this may not be practical since manufacturing optimizationand cost considerations often limit the variations in doping profilesthat can be used in the manufacturing sequence for a particularsemiconductor device or IC. This is especially a concern when bothbipolar devices and field effect (e.g., CMOS) devices are being formedin the same manufacturing process. Accordingly, a need continues toexist for improved bipolar transistors and methods for manufacturing thesame.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 shows a simplified cross-sectional view about a centerline of animproved bipolar transistor according to an embodiment of the presentinvention;

FIG. 2 shows a simplified cross-sectional view about a centerline of animproved bipolar transistor according to another embodiment of thepresent invention;

FIG. 3 shows a simplified cross-sectional view about a centerline of animproved bipolar transistor according to still another embodiment of thepresent invention;

FIG. 4 shows a simplified cross-sectional view about a centerline of animproved bipolar transistor according to yet another embodiment of thepresent invention;

FIG. 5 shows a simplified cross-sectional view of the transistor of FIG.1 wherein both halves of the transistor about the centerline areincluded; and

FIGS. 6-14 show simplified cross-sectional views of the bipolartransistors of FIGS. 1-5 during various stages of manufacture.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, or the following detailed description.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the invention. Additionally, elements in thedrawing figures are not necessarily drawn to scale. For example, thedimensions of some of the elements or regions in the figures may beexaggerated relative to other elements or regions to help improveunderstanding of embodiments of the invention.

The terms “first,” “second,” “third,” “fourth” and the like in thedescription and the claims, if any, may be used for distinguishingbetween similar elements or steps and not necessarily for describing aparticular sequential or chronological order. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances such that the embodiments of the invention describedherein are, for example, capable of operation or arrangement insequences other than those illustrated or otherwise described herein.Furthermore, the terms “comprise,” “include,” “have” and any variationsthereof, are intended to cover non-exclusive inclusions, such that aprocess, method, article, or apparatus that comprises a list of elementsor steps is not necessarily limited to those elements, but may includeother elements or steps not expressly listed or inherent to suchprocess, method, article, or apparatus. The term “coupled,” as usedherein, is defined as directly or indirectly connected in an electricalor non-electrical manner. As used herein the terms “substantial” and“substantially” mean sufficient to accomplish the stated purpose in apractical manner and that minor imperfections, if any, are notsignificant for the stated purpose.

As used herein, the term “semiconductor” and the abbreviation “SC” areintended to include any semiconductor whether single crystal,poly-crystalline or amorphous and to include type IV semiconductors,non-type IV semiconductors, compound semiconductors as well as organicand inorganic semiconductors. Further, the terms “substrate” and“semiconductor substrate” and “SC substrate” are intended to includesingle crystal structures, polycrystalline structures, amorphousstructures, thin film structures, layered structures as for example andnot intended to be limiting, semiconductor-on-insulator (SOI)structures, and combinations thereof

For convenience of explanation and not intended to be limiting,semiconductor devices and methods of fabrication are described hereinfor silicon semiconductors, but persons of skill in the art willunderstand that other semiconductor materials may also be used.Additionally, various device types and/or doped SC regions may beidentified as being of N type or P type, but this is merely forconvenience of description and not intended to be limiting, and suchidentification may be replaced by the more general description of beingof a “first conductivity type” or a “second, opposite conductivity type”where the first type may be either N or P type and the second type isthen either P or N type. Various embodiments of the invention will beillustrated for NPN bipolar transistors, but this is merely forconvenience of description and is not intended to be limiting. Personsof skill in the art will understand that PNP transistors and othersemiconductor devices and circuits embodying either or both NPN and PNPcombinations may be provided by appropriate interchange of conductivitytypes in the various regions.

Adjusting doping profiles to suit particular device needs is astraightforward way to optimize device properties. However, adding ormodifying process steps to provide a desired bipolar devicecharacteristic generally adds cost and complexity. This is especially ofconcern for Bi-CMOS processes used to manufacture bipolar and fieldeffect devoices at the same time on the same substrate. Accordingly, aneed exists for improved bipolar transistors and methods formanufacturing the same: (i) that are adapted to being “tuned” to suitparticular applications, (ii) that accommodate a wide range of designspace with little or no compromise of other properties, and (iii) thatcan be accomplished by layout adjustments without adding orsignificantly modifying process steps that would increase manufacturingcost. With the embodiments described below, bipolar device propertiescan be substantially modified just by layout adjustments. Theseembodiments can provide a wide design space without adding process cost.Thus, doping and masking steps can be shared with other devices on thewafer or chip without the bipolar transistors imposing undesirableconstraints on process optimization.

It has been discovered that broadly tunable high gain, high breakdownvoltage, bipolar transistors with significant high-side capability canbe provided by incorporating multiple emitter-base junctions havingdifferent emitter-base junction depths, desirably combined with adepletable collector structure. In preferred embodiments different basedoping is provided under the various emitter-base junctions. Additionaltuning of the device properties can be achieved by varying theemitter-collector lateral separation.

FIGS. 1-4 show simplified cross-sectional views laterally fromcenterline 19 of improved bipolar transistors 20-1, 20-2, 20-3, 20-4,(collectively transistors 20) according to various embodiments of thepresent invention, and FIG. 5 shows, by way of example, transistor 20-1of FIG. 1 with both halves of the transistor about centerline 19included. The drawings illustrate the conductivity types appropriate forNPN transistors, but those of skill in the art will understand that thisis merely for convenience of description and not intended to belimiting. Also for convenience of description and not intended to belimiting, the convention is adopted of referring to the variousembodiments depicted in FIGS. 1-5 and various regions in suchembodiments, by using a primary reference number to identify aparticular element that may be present in all of the embodimentsfollowed by a suffix to identify the particular embodiment to which itapplies. For example, in the embodiment of FIG. 1, transistor 20 isidentified as transistor 20-1 and in the embodiment of FIG. 2 transistor20 is identified as transistor 20-2. Correspondingly, the collectorburied layer (BL) is identified as BL 30-1 in transistor 20-1 of FIG. 1and BL 30-2 in transistor 30-2 of FIG. 2. Where it is intended to pointout that a particular element may be different in various embodiments, asuffix is used and where a particular element may be substantiallysimilar in various embodiments, no suffix is used or the same suffix maybe used in multiple embodiments for those elements that can besubstantially similar in such embodiments. This convention is generallyfollowed throughout and is intended for convenience of explanation andnot limitation; in particular, it is not intended to infer that elementswith no suffixes or with common suffixes must be unchanged fromembodiment to embodiment or similar in all such embodiments.

Referring now to FIGS. 1-5, transistors 20 comprise substrate layer 21having lower or bottom surface 22. Substrate layer 21 may be asemiconductor (SC) or dielectric substrate. In a preferred embodimentfor NPN transistors, substrate layer 21 is N type but may be of otherconductivity type in other embodiments for either NPN or PNP transistorsor may be an insulating substrate. In a preferred embodiment substratelayer 21 is a semiconductor (SC) and buried oxide (BOX) layer 24overlies substrate layer 21. Above BOX layer 24 is semiconductor (SC)layer 28, preferably an epitaxial (EPI) layer extending to upper surface23. For convenience of description and not intended to be limiting,layer 28 is henceforth referred to as “EPI layer 28” or “EPI 28”, butpersons of skill in the art will understand that SC layer 28 formed byother means may also be used and the designation “EPI” as used hereinfor layer 28 is intended to include such other means of formation.Reference number 29 in FIGS. 1-5 and 8-14 is intended to refer to thecombination of substrate layer 21 and layer 28, with or without BOXlayer 24 and/or buried layer 30, as illustrated further in connectionwith FIGS. 7-8. For convenience of description the terms “substrate 29”and “substrate (29)” are used when referring to this combination. As isexplained in more detail in connection with FIGS. 6-14, layer 28 isdesirably formed from two stacked EPI layers, EPI-1 layer 25 and EPI-2layer 26 that join at interface 27, but a single SC layer may also beused in other embodiments.

Lying within EPI layer 28 at or near interface 27 is (e.g., N type)buried layer (BL) 30 of lateral width 31. Extending from (e.g., N type)BL 30 substantially to surface 23 is (e.g., N type) WELL region 34.Collector contact (e.g., N+) 35 is provided at surface 23 in WELL region34, and is coupled to collector terminal 92. Extending into EPI layer 28from surface 23 is (e.g., N type) first emitter region 40. Emittercontact (e.g., N+) 41 is provided at surface 23 in Ohmic contact with(e.g., N type) first emitter region 40 and is coupled to emitterterminal 90. Also extending into EPI layer 28 from surface 23 areshallow trench isolation (STI) regions 55 of, for example, a dielectricsuch as silicon oxide. Toward the right in FIGS. 1-4 and at both leftand right in FIG. 5, are (e.g., dielectric) deep trench isolation (DTI)regions 60, also typically of silicon oxide, extending in preferredembodiments substantially from surface 23 to substrate layer 21. Incombination with BOX layer 24, DTI regions 60 serve to isolatetransistors 20 from other devices or regions on substrate layer 21. In apreferred embodiment, conductive (e.g. poly-semiconductor) core 61 isprovided in DTI regions 60 but may be omitted in other embodiments.Techniques for creating such STI and DTI regions are well known in theart. STI regions 55 are particularly identified as STI regions 55-11,55-12, 55-13 in transistor 20-1 of FIGS. 1 and 5, and as STI regions55-21, 55-22, 55-23 in transistors 20-2, 20-3, 20-4 of FIGS. 2-4.

Underlying surface 23 at the left of STI regions 55-11 and 55-21 are(e.g., N type) first emitter regions 40 with (e.g., N+) of depth 46 fromsurface 23 and with emitter contacts 41 at surface 23. Underlying STIregions 55-11 and 55-21 are (e.g., N type) second emitter regions 43 oflateral width 44, particularly identified as (e.g., N type) secondemitter regions 43-1 of width 44-1 in FIGS. 1 and 5 and (e.g., N type)second emitter regions 43-2 of width 44-2 in FIGS. 2-4, and collectivelyreferred to as second emitter regions 43. Underlying (e.g., N type)first emitter regions 40 are (e.g., P type) first base regions 50 thatform (e.g., NP) first emitter-base junctions 45 at depth 46 from surface23. Underlying (e.g., N type) second emitter regions 43 are (e.g., Ptype) second base regions 51 that form (e.g., NP) second emitter-basejunctions 47 at depth 48 from surface 23. It will be understood by thoseof skill in the art that junctions 45 and 47 may be NP or PN junctionsdepending upon whether NPN or PNP transistors are being formed. Secondbase regions 51 are particularly identified as regions 51-1 in device20-1 of FIGS. 1 and 5, and regions 51-2 in devices 20-2, 20-3, 20-4 ofFIGS. 2-4, and collectively referred to as second base region 51. Ingeneral, (e.g., P type) first base regions 50 have about the samelateral extent as (e.g., N type) first emitter regions 40, and (e.g., Ptype) second base regions 51 have about the same lateral extent as(e.g., N type) second emitter regions 43, but other dimensions may alsobe used in other embodiments.

Laterally adjacent to second emitter regions 43 and in Ohmic contactwith (e.g., P type) second base regions 51 are (e.g., P type) WELLregions 52 extending from surface 23 to a depth sufficient to provide arelatively low resistance Ohmic connection to second base regions 51.Base contacts (e.g., P+) 53 extend from surface 23 into (e.g., P type)WELL regions 52 and are coupled to base terminal 91. Below STI regions55-12 of device 20-1 of FIGS. 1 and 5 and 55-22 of device 20-2, 20-3,20-4 of FIGS. 2-4 are (e.g., P type) regions 261 of width 57-1 in FIGS.1 and 5 and 57-2 of FIGS. 2-4, also referred to collectively as width57. Regions 261 of lateral width 57 lie substantially between (e.g., Ptype) WELL regions 52 and (e.g., N type) WELL regions 34. In transistors20, to the right of (e.g., N type) WELL regions 34, between WELL regions34 and deep trench isolation regions 60 lie (e.g., P type) regions 262.As is subsequently explained, the doping of regions 261, 262 isdetermined substantially by the doping of EPI-2 layer 26.

Depth 46 of first emitter-base junction 45 and depth 48 of secondemitter-base junction 47 are different, as are the depth and doping ofunderlying (e.g., P type) base regions 50 and 51. These differences incombination with the ability to vary lateral emitter-collector spacingand overlap allow great flexibility in “tuning” the transistors to suitvarious applications. In preferred embodiments, first emitter-basejunctions 45 are deeper than second emitter-base junctions 47, that is,depth 46 exceeds depth 48. Also in preferred embodiments, the doping offirst base regions 50 is less than the doping of second base regions 51.

Transistor 20-1 of FIGS. 1 and 5 differ from transistors 20-2, 20-3,20-4 of FIGS. 2-4 in that collector lateral width 31-1 is larger thancollector lateral width 31-2 and the amount by which collectors 30-1 and30-2 do or do not underlie second emitters 43-1, 43-2 are different. Byway of example and not intended to be limiting, in device 20-1 of FIGS.1 and 5, BL collector 30-1 underlies second emitter 43-1 by overlapdistance 33-1 and is laterally spaced distance 32-1 away from firstemitter 40. Separating collector BL 30-1 from emitter 40 by lateraldistance 32-1 significantly improves the Early Voltage andemitter-collector breakdown voltage (BVceo) of transistors 20-1 of FIGS.1 and 5 with little adverse effects on other key device parameters. Byway of example and not intended to be limiting, in devices 20-2, 20-3,20-4 of FIGS. 2-4, BL collector 30-2 does not underlie second emitter43-2, is spaced lateral distance 33-2 therefrom and is laterally spaceddistance 32-2 away from first emitter 40. Such configuration furtherimproves the Early Voltage of transistors 20-2, 20-3 and 20-4 in FIGS.2-4.

Transistor 20-3 of FIG. 3 differs from transistor 20-2 of FIG. 2 in thatconductive (e.g., doped poly) gate 65 is provided over a portion of STIregion 55-21 and electrically coupled, for example, to base contact 53by connection 66 to act as a field plate. In other embodiments, gate 65may be connected to other dynamic or static potential sources. Eitherarrangement is useful. Bias on conductive gate 65 is expected tomodulate the carrier concentration of vertical emitter-base junction 49laterally located between emitter 43-2 of FIG. 3 and PWELL 52 underneathSTI 55-21. This can provide further improvement in the emitter-basejunction breakdown voltage (BVebo).

Transistor 20-4 of FIG. 4 differs from transistor 20-2 of FIG. 2, inthat emitter portion 41-4 of (e.g., N+) emitter contact 41 extends over(e.g., P type) portion 263 of EPI 28, so that (e.g., N+) portion 41-4 ofemitter contact 41 acts as a further or third emitter forming further orthird (e.g., NP) emitter-base junction 59 with (e.g., P type) portion263 that acts as a further or third base region. Forming thirdemitter-base junction 59 in this manner can further improve the currentgain of transistor 20-4 due to the relatively high doping concentrationof third emitter 41-4 and the relatively low doping concentration inthird base portion 263. Third base portion 263 is formed from EPI-2layer 26 (e.g., see FIG. 8) and, as will be subsequently explained, isrelatively lightly doped so that it can be fully depleted when theemitter-base junctions are reverse biased. The complete depletion ofportion 263 takes place before the breakdown of PN junction 59 betweenthird emitter 41-4 and third base portion 263. Accordingly, includingportion 41-4 can improve current gain without impairing breakdownvoltages and other key device parameters. While the various embodimentsof transistors 20 illustrated in FIGS. 1-4 show particular combinationsof elements, sizes, shapes, spacings, etc., these are intended by way ofexample and not limitation. Variations in lateral sizes, shapes andspacings 31, 32 and 33 may be combined with conductive gate 65 and/orwith third emitter 41-4 to provide a wide range of device properties, tofacilitate “tuning” bipolar transistors 20 to suit a particularapplications (e.g., achieving a desired gain and/or Early voltage)without significant adverse effect on other desirable properties (e.g.,while still achieving a desired breakdown voltage BVebo and/or BVcboand/or BVceo, and/or achieving a desired high-side capability, etc.).This is very useful.

FIGS. 6-14 show simplified cross-sectional views of bipolar transistors20 of FIGS. 1-5 during various stages 406-414 of manufacture. Structures506-514 result from manufacturing stages 406-414 respectively. Forconvenience of explanation and not intended to be limiting, thestructure associated with FIGS. 1 and 5 is illustrated in connectionwith manufacturing stages 406-414 of FIGS. 6-14, but persons of skill inthe art will understand that the embodiments of transistors 20-2, 20-3,20-4, etc., illustrated in FIGS. 2-4 may also be formed usingsubstantially similar manufacturing stages, for example, merely withappropriate mask changes in the case of transistors 20-2 and 20-4 and/orby additionally providing conductive gate 65 in the case of transistor20-3, or any combination thereof Adding conductive gate 65 generallydoes not require added process steps since providing conductive layersor regions of a particular geometry is a common feature of essentiallyall SC manufacturing processes and may be accomplished by simple maskchanges without added process steps. Because FIGS. 6-14 are intended byway of example and not limitation to illustrate the fabrication ofdevice 20-1 of FIGS. 1 and 5, the custom is adopted of referring tovarious regions by those reference numbers associated with FIGS. 1 and5, even though it will be understood that analogous regions in FIGS. 2-4are fabricated in substantially the same way except for mask changes.Thus, references to regions 43-1, 51-1, 55-11, 55-12, 55-13, etc., areintended to include analogous regions 43-2, 51-2, 55-21, 55-22, 55-23,etc. Unless otherwise noted, doping of various regions within devices 20is preferably accomplished either: (a) during epitaxial growth ofvarious layers, or (b) by ion implantation, or (c) by a combinationthereof, but this is not intended to be limiting and other doping meansmay also be used. For ion implantation, photoresist is a suitable maskmaterial and unless otherwise noted is used throughout, but other maskmaterial may also be used.

Referring now to manufacturing stage 406 of FIG. 6, substrate layer 21is provided having thereon BOX layer 24 and, for example, EPI-1 layer25. This combination is also referred to as “substrate 29′ ” or“substrate (29′)” analogous to the use of “substrate 29” explainedearlier, but with the prime 0 added to indicate that it is a precursorto substrate 29, e.g., lacking layer 26. Substrate layer 21 may besemiconductor (SC) or insulator, and if SC, either N or P type. In apreferred embodiment, where transistor 20 is an NPN transistor, thensubstrate layer 21 is preferably N type with doping usefully in therange of about 5E14 to 1E18 cm⁻³, preferably about 1E15 to 5E15 cm⁻³,but higher or lower doping levels can also be used. BOX layer 24 isconveniently formed of oxide but other dielectrics can also be used. BOXlayer 24 has thickness 241 desirably in the range of about 0.1 to 1.0micrometers, preferably in the range of about 0.2 to 0.6 micrometers,but thicker or thinner layers can also be used. SC layer 25 of thickness251, preferably formed epitaxially (hereafter EPI-1 layer 25 or EPI-125), is provided above BOX layer 24. Where transistors 20 are to be NPNtransistors, EPI-1 layer 25 is preferably P type. Thickness 251 of EPI-1layer 25 is usefully in the range of about 0.2 to 6.0 micrometers,preferably in the range of about 0.4 to 3.5 micrometers, but thicker orthinner layers can also be used. EPI-1 layer 25 usefully has doping inthe range of about 5E14 to 1E16 cm⁻³, preferably about 1E15 to 5E15cm⁻³, but higher or lower doping levels can also be used. EPI-1 layer 25has upper surface 27. Structure 506 results.

Referring now to manufacturing stage 407 of FIG. 7, mask 600 havingopening 601 is provided on surface 27 of structure 506. Opening 601 haslateral dimensions and location corresponding approximately to thedesired eventual location of buried layer (BL) collector 30. Implant 700is provided through opening 601 to form (e.g., N type) doped region 30″of thickness 602. Implant 700 is, for example, phosphorus with a doseusefully in the range of about 8E11 to 5E13 cm⁻² and preferably in therange of about 1E12 to 6E12 cm⁻², at energies usefully in the range ofabout 30 KeV to 2 MeV and preferably in the range of about 40 KeV to 200KeV, but other impurities and larger or smaller doses and other energiescan also be used. Structure 507 results.

Referring now to manufacturing stage 408 of FIG. 8, mask 600 ofstructure 507 is removed and SC layer 26 (hereafter EPI-2 layer 26 orEPI-2 26) of thickness 261 having upper surface 23 is provided oversurface 27 of EPI-1 layer 25, preferably by epitaxial growth, but otherformation techniques can also be used. EPI-2 layer 26 is preferably Ptype where transistors 20 are NPN transistors, with doping usefully inthe range of about 5E14 to 1E16 cm⁻³ and preferably in the range ofabout 1E15 to 5E15 cm⁻³, but other doping densities may also be used. Ina preferred embodiment, the doping of EPI-1 layer 25 and EPI-2 layer 26are substantially similar, but different doping levels may also be usedin EPI-1 layer 25 and EPI-2 layer 26. Thickness 261 of EPI-2 layer 26 isusefully in the range of about 1.0 to 6.0 micrometers, preferably in therange of about 2.0 to 4.0 micrometers. Stated another way, thickness 281of EPI layer 28 is usefully in the range of about 3 to 12 micrometers,preferably in the range of about 4 to 8 micrometers. During the heattreatment associated with the formation of SC layer 26, doped region 30″diffuses thereby forming BL collector 30 of thickness 303 generallygreater than thickness 602 of FIG. 7 where out-diffusion from dopedregion 30″ occurs. BL collector 30 resulting from the foregoing stepshas (e.g., N type) doping concentration usefully in the range of about1E15 to 1E18 cm⁻³, more conveniently in the range of about 1E16 to 1E17cm⁻³, and preferably in the range of about 2E16 to 8E16 cm⁻³. It will benoted that (e.g., N type) BL collector 30 is surrounded above and belowby (e.g., P type) regions or layers 25, 26 of opposite conductivitytype. This arrangement in combination with the relatively low doping ofBL collector 30 compared to typical BL doping, facilitates depletion ofBL collector 30 during operation of completed transistor 20. Thisspreads the collector electric field and assists in maximizing thebreakdown voltage of transistors 20. The BOX architecture is leveragedto prevent any undesirable currents between base and the substrate, asthe collector does not serve as an isolating element in contrast toprior art. While forming doped region 30″ prior to forming EPI-2 layer26 is convenient and allows for relatively precise control of the dopingof layers 26 and 25 above and below BL collector 30, any other means ofproviding BL 30 may be used in other embodiments, as for example and notintended to be limiting, implanting BL collector 30 after formation ofSC layer 28 as a whole. Such alternative arrangements are useful.Structure 508 results.

Referring now to manufacturing stage 409 of FIG. 9, STI regions 55 andDTI region 60 with conductive core 61 are provided extending into SC EPIlayer 28 from surface 23. STI regions 55-11, 55-12, 55-13 areparticularly illustrated in FIG. 9, but persons of skill in the art willunderstand that STI regions 55-21, 55-22, 55-23, etc., could likewise beformed in EPI 28 from surface 23. STI regions 55 are preferably ofsilicon oxide, formed for example, by high density plasma chemical vapordeposition and have thickness 551 usefully in the range of about 0.2 to0.8 micrometers and preferably in the range of about 0.3 to 0.6micrometers, but larger or smaller thicknesses and other materials mayalso be used. As will be subsequently explained in connection withmanufacturing stage 410, STI regions 55-11 and 55-21 are used topartially attenuate the implant used to form second emitters 43 andunderlying second base regions 51, therefore, it is desirable that thematerial and thickness of such STI regions be chosen with such implantattenuation in mind (along with the intended implant conditions).Persons of skill in the art will understand how to do this based uponthe desired doping of such regions described in connection withmanufacturing stage 410. Techniques for forming dielectric STI regions55 and DTI regions 60 (with or without conductive core 61) are wellknown in the art. Structure 509 results from manufacturing stage 409.

Referring now to manufacturing stage 410 of FIG. 10, mask 610 havingopening 611 is applied over surface 23 of structure 509 of FIG. 9.Implant 710 is provided through opening 611 in mask 610. Implant 710 isdesirably a chain implant, adapted to provide in a single implant step,both first emitter regions 40 and second emitter regions 43 (e.g., 43-1in device 20-1 and 43-2 in device 20-2, 20-3, 20-4) using a relativelyshallow (e.g., N type) implant, and provide first base regions 50 andsecond base regions 51 (e.g., 51-1 in device 20-1 and 51-2 in device20-2, 20-3, 20-4) using a relatively deeper (e.g., P type) implant. ForNPN transistors, phosphorous is a non-limiting example of a suitabledopant for the N type implant and boron is a non-limiting example of asuitable dopant for the P type implant. Where other transistor types(e.g., PNP) are desired, persons of skill in the art will understand howto chose appropriate dopants. Multiple N-type implants may be used togenerate the desired doping profile depending on the available thermalbudget for diffusion. In forming second emitters 43 and second bases 51,implants with sufficiently low energy will be blocked, for example, bySTI region 55-11, while higher energy implants can penetrate STI region55-11 and contribute to doping of the SC underneath STI region 55-11. Infirst emitter region 40, lower energy implants as part of the implantchain may diffuse downwards, adding to the final concentration in firstemitter region 40. In contrast, low energy dopants stopped in STI region55-11 during implantation will remain trapped therein due to their muchlower diffusivity in the dielectric material (e.g., oxide) of which STIregion 55-11 is formed. For STI thickness 551 (see FIG. 9) in the rangeof about 0.3 to 0.6 micrometers, high energy phosphorous implants aredesirably made at energies in the range of about 400 KeV to 1 MeV, so asto provide second emitter region 43-1, and low energy phosphorusimplants are desirably made at energies below 400 keV for so as toincrease the doping near surface 23 in first emitter region 40, buthigher or lower energies may also be used depending upon the desiredjunction depths and the thickness of STI region 55-11.

Both, N type and P type high energy implants are preferably performed ata 0 degree tilt for maximal channeling of ions for emitter region 40 andbase region 50. In contrast, the thick oxide layer of the STI suppresseschanneling of implant ions for emitter region 43-1 and base region 51-1,resulting in a different emitter-base depth profile in the secondemitter region for the same implant chain as used for the first emitterregion. It is well known in the art, that channeling for high energyions will result in a double peak profile consisting of a shallowerprimary peak and a deeper channeling peak with channeling tail. While,in preferred embodiments, the P type profile of first base region 50exhibits such a double peak structure, the second base region profile51-1 only shows a single peak profile containing essentially the samedose as implanted into region 50 at about the same depth as the primarypeak of region 50 as channeling is suppressed due to the STI. Therefore,after implanting the high energy P type impurities, there is a broaddouble peak profile of lower peak concentration in region 50, and asharper single peak profile with higher peak concentration in region51-1. The higher energy N type implants are performed subsequently. Inpreferred embodiments, the channeling tail of the higher energy portionof the N type implants in 40 counter dopes the shallower portions of theunderlying base region 50 leading to decreased net base doping anddeeper emitter-base junction in the first base region 50. In contrast,channeling is suppressed under the STI in region 43-1 such that the Ntype profile does not significantly overlap with the underlying P typebase profile resulting in a shallower emitter-base junction and heavierbase doping in the second base region 51-1 compared to the first.

First emitter region 40 (e.g., N type) desirably has a first emitterdoping concentration in the range of about 1E16 to 1E18 cm⁻³, andpreferably about 8E16 cm⁻³. First base region 50 (e.g., P type)desirably has a first base doping concentration in the range of about1E16 to 1E18 cm⁻³, and preferably about 4E16 cm⁻³. PN junction 45 formedbetween first emitter region 40 and first base region 50 has depth 46conveniently in the range of about 1 to 3 micrometers beneath surface 23and preferably about 1.7 micrometers beneath surface 23, but deeper andshallower junctions and higher and lower doping concentrations may alsobe used. When STI region 55-11 is formed of silicon oxide about 0.4micrometers thick, second emitter region 43-1 (e.g., N type) desirablyhas a second emitter doping concentration beneath STI region 55-11 inthe range of about E16 to 1E18 cm⁻³, and preferably about 5E16 cm⁻³.Second base region 51-1 (e.g., P type) is located beneath second emitterregion 43-1 and has second base doping concentration in the range ofabout 1E16 to 1E18 cm⁻³, and preferably about 6E16 cm⁻³. PN junction 47formed between second emitter region 43-1 and second base region 51-1has junction depth 48 usefully in the range of about 1 to 3 micrometersbeneath surface 23 and preferably about 1.5 micrometers beneath surface23, but deeper and shallower junctions and higher and lowerconcentrations may also be used. Stated another way, it is desirablethat second emitter region 43-1 have a second emitter dopingconcentration that is less than the first emitter doping concentrationby a factor of about 1 to 100 and preferably by a factor of about 1.5.It is further desirable that second base region 51-1 has a second basedoping concentration that is greater than the first base dopingconcentration by a factor of about 1 to 10 and preferably by a factor ofabout 1.5. Structure 510 results from manufacturing stage 410.

Referring now to manufacturing stage 411 of FIG. 11, mask 610 of stage410 is desirably removed and replaced by mask 620 having opening 621.Implant 720 (e.g., P type) is applied through opening 621 to form WELL52 (e.g., P type) extending from surface 23 toward collector BL 30sufficiently to provide a relatively low resistance Ohmic contact tosecond base region 51-1, but deeper or shallower implants may also beused. Opening 621 for forming (e.g., P type) WELL 52 may laterallyoverlap regions 43-1 and 51-1. WELL 52 usefully has (e.g., P type)doping in the range of about 1E17 to 5E18 cm⁻³ and preferably has dopingin the range of about 2E17 to 2E18 cm⁻³. Structure 511 results.

Referring now to manufacturing stage 412 of FIG. 12, mask 620 of stage411 is replaced by mask 630 having opening 631. Implant 730 (e.g., Ntype) is provided through opening 730 to form WELL 34 (e.g., N type)extending substantially from surface 23 to BL collector 30 with which itmakes Ohmic contact, so as to electrically couple BL collector 30 tosurface 23. Structure 512 results. Referring now to manufacturing stage413 of FIG. 13, mask 630 of stage 412 is replaced by mask 640 havingopenings 641, 642. Implant 740 (e.g., N type) is provided throughopenings 641, 642 to form (e.g., N+) collector contact region 35 underopening 641 making Ohmic contact to (e.g., N type) WELL 34 and to form(e.g., N+) emitter contact region 41 under opening 642 making Ohmiccontact to first emitter region 40. Structure 513 results. In the caseof device 20-4 of FIG. 4, opening 642 extends further to the left ofregion 40 so that (e.g., N+) portion 41-4 makes junction contact tounderlying portion 263 of EPI-2 layer 26, as shown in FIG. 4. Referringnow to manufacturing stage 414 of FIG. 14, mask 640 is removed andreplaced by mask 650 having opening 651. Implant 750 (e.g., P type) ismade through opening 651 to form (e.g., P+) base contact to (e.g., Ptype) WELL 52. Structure 514 results. Other than removing mask 650 andproviding conventional interconnections to contact regions 35, 41, 53(depending on how the device is being coupled to other circuit elements)and conventional passivation layers (not shown) above surface 23, device20 is substantially complete.

For transistors 20-1, 20-2, and 20-3, the majority of carriers (e.g.,electrons) are injected from first emitter region 40 into first baseregion 50 in an approximately vertical direction, which gives rise to ahigh current gain. In transistor 20-4, most electrons are injected fromemitter contact region 41-4 into lightly doped base region 263, whichcan provide an even higher current gain than that attained intransistors 20-1, 20-2, 20-3. The Early voltage of transistors 20-1,20-2, and 20-3 is determined by two bipolar transistors connected inparallel. One bipolar transistor is formed by first emitter 40, firstbase 50 and EPI 28, and collector 30 at lateral distance 32-1. Anotherbipolar transistor is formed by second emitter 43-1, second base 51-1 intransistor 20-1, second base 51-2 and EPI 28 between region 51-2 intransistor 20-2 and 20-3, and BL collector 30. For transistor 20-4,portion 41-4 also plays an important role in determining the EarlyVoltage. When BL collector 30 is placed at least partially underlyingemitter 43-1, as for example in FIG. 1, high gain but relatively lowEarly Voltage are obtained. When BL collector 30 is pulled laterallyaway from emitter 43-1, as for example in FIGS. 2-4, the current gain isreduced due to the larger base width but the Early Voltage is increased.Accordingly, BL collector 30 of bipolar transistors 20-1, 20-2, 20-3,20-4, etc., can be placed in various lateral positions relative to firstand second emitter regions 40, 43, thereby permitting a wide range ofdevice characteristics to be obtained to suit specific applications,merely by mask changes without adding further process steps. Intransistors 20-1, 20-2, 20-3, 20-4, emitter 43 has a moderate dopingconcentration, and it can be depleted by extrinsic base WELL 52proximate junction 49 and by intrinsic base 51 underneath emitter region43, which significantly improves BVebo. A field plate located on STIregion 55-11 above junction 49 (e.g., as shown for device 20-3 of FIG.3) may be used to assist the depletion and further improve thebreak-down voltage. In addition, because of the relatively light dopingof BL collector 30, it can be fully depleted from all directions by thesurrounding P-regions biased with the base (or other) potential when thecollector voltage is high relative to other terminals. This combinationof features significantly improves BVcbo and BVceo and providestransistors 20-1, 20-2, 20-3, 20-4 in FIGS. 1-5 a high side capability.Accordingly, transistors 20-1, 20-2, 20-3, 20-4 facilitate tuning thebipolar transistor characteristics by, among other things, varying theposition of the BL collector 30 relative to other elements withoutsacrificing the breakdown voltages and high side capability. This isvery useful.

According to a first embodiment, there is provided a bipolar transistor(20), comprising, a semiconductor substrate (29) having a first surface(23), a first emitter region (40) of a first conductivity type in thesemiconductor substrate (29) having a first emitter region dopingconcentration, a first base region (50) of a second, oppositeconductivity type in the semiconductor substrate (29) underlying thefirst emitter region (40, 41-4) and having a first base region dopingconcentration, the first base region (50) forming a first PN or NPjunction (45, 59) with the first emitter region (40) at a first depth(46) from the first surface (23), a second emitter region (43) of thefirst conductivity type in the semiconductor substrate (29) having asecond emitter region doping concentration and Ohmically coupled to thefirst emitter region (40), a second base region (51) of the secondconductivity type in the semiconductor substrate (29) underlying thesecond emitter region (43) and having a second base region dopingconcentration different than the first base doping concentration, thesecond base region (51) forming a second PN or NP junction (47) with thesecond emitter region (51) at a second depth (48) from the first surface(23), and a buried layer collector region (30) of the first conductivitytype in the substrate (29) underlying the first surface (23) andlaterally spaced a third distance (32) from the first emitter region(40). According to a further embodiment, the first emitter region (40)and the second emitter region (43) are laterally adjacent. According toa further embodiment, the first base region (50) and the second baseregion (51) are laterally adjacent. According to a still furtherembodiment, the first depth is greater than the second depth. Accordingto a yet further embodiment, the second emitter region (43) lieslaterally at least partly between the first emitter region (40) and theburied layer collector region (30). According to a still yet furtherembodiment, the second base region (51) lies laterally at least partlybetween the first base region (50) and the buried layer collector region(30). According to a yet still further embodiment, the transistor (20)further comprises a third emitter region (41-4) of a third emitterdoping concentration and a third base region (263) of a third basedoping concentration, wherein the third emitter region (41-4) is coupledto the first emitter region (40) and the third base region is coupled tothe first base region (50) and the third base doping concentration isless than the first base doping concentration. According to anotherembodiment, the second emitter region (43) has a lateral extent (44)greater than the third distance (32). According to still anotherembodiment, the second emitter region (43) has a lateral extent (44)less than the third distance (32).

According to a second embodiment, there is provided a method for forminga bipolar transistor, comprising, providing a semiconductor containingsubstrate (29) with an upper surface (23), and having therein a buriedlayer collector region (30) of a first conductivity type located belowthe upper surface (23) and of a buried layer collector region dopingconcentration, wherein a first portion (25) of the substrate locatedbelow the buried layer collector region (30) and a second portion (26)of the substrate located above the buried layer collector region (30)are of a second, opposite conductivity type, forming during one or morefirst doping steps in the second portion (26) of the substrate (29),first (40) and second (43) adjacent emitter regions of the firstconductivity type near the first surface, the first emitter regionextending substantially to the first surface and laterally separatedfrom the buried layer collector region (30), and forming during one ormore second doping steps in the second portion (26) of the substrate(29), first (50) and second (51) adjacent base regions of the secondconductivity type, respectively, beneath the first (40) and second (43)emitter regions, wherein the first base region (50) has a first baseregion doping concentration and the second base region (51) has a secondbase region doping concentration different than the first base regiondoping concentration, and the first emitter region (40) and first baseregion (50) providing a first NP or PN junction (45) at a first depth(46) beneath the upper surface (23) and the second emitter region (43)and the second base region (51) providing a second NP or PN junction(47) at a second depth (48) beneath the upper surface (23). According toa further embodiment,. the method further comprises providing at leastone isolation region (55-11, 55-21) substantially at the upper surface(23), wherein the second emitter region (43) and the second base region(51) substantially underlie the at least one isolation region (55-11,55-21). According to a still further embodiment, the one or more firstdoping steps and the one or more second doping steps are performedduring one or more chain implants of dopants of opposite conductivitytype. According to a still further embodiment, the buried layercollector region (30) and the second base region (51) are laterallyarranged so that the buried layer collector region (30) extendsunderneath the second base region (51). According to a yet furtherembodiment, the buried layer collector region (30) and the second baseregion (51) are laterally arranged so that the buried layer collectorregion (30) does not extend underneath the second base region (51).According to a still yet further embodiment, the method furthercomprises forming a third emitter region (41-4) and third base region(263) having an NP or PN junction (59) there between, laterally arrangedon a side of the first emitter region (40) opposite the second emitterregion (43).

According to a third embodiment, there is provided a bipolar transistor(20) having a first surface (23), comprising, first (45) and second (47)connected emitter-base junctions having, respectively, first (46) andsecond (48) different junction depths beneath the first surface (23),and a buried layer collector region (30) having a third depth (304)beneath the first surface (23), the third depth (304) being greater thanthe first (46) or second (48) depths, wherein the buried layer collectorregion (30) does not underlie the first (45) emitter-base junction.According to a further embodiment, the buried layer collector region(30) underlies at least a portion of the second emitter-base junction(47). According to a still further embodiment, the transistor (20)further comprises an isolation region (55-11, 55-21) overlying thesecond emitter-base junction (47) but not overlying the firstemitter-base junction (45). According to a yet further embodiment, thetransistor (20) still further comprises a conductive gate (65) on theisolation region (55-21) overlying a portion (49) of the secondemitter-base junction (47). According to a still yet further embodiment,the conductive gate (65) is electrically coupled to a base region (51)of the second emitter-base junction (47).

While at least one exemplary embodiment and method of fabrication hasbeen presented in the foregoing detailed description of the invention,it should be appreciated that a vast number of variations exist. Itshould also be appreciated that the exemplary embodiment or exemplaryembodiments are only examples, and are not intended to limit the scope,applicability, or configuration of the invention in any way. Rather, theforegoing detailed description will provide those skilled in the artwith a convenient road map for implementing an exemplary embodiment ofthe invention, it being understood that various changes may be made inthe function and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

What is claimed is:
 1. A method for producing a bipolar transistor,comprising: providing a semiconductor-containing substrate with an uppersurface, and having therein a buried layer collector region of a firstconductivity type located below the upper surface and of a buried layercollector region doping concentration, wherein a first portion of thesubstrate located below the buried layer collector region and a secondportion of the substrate located above the buried layer collector regionare of a second, opposite conductivity type; during one or more firstdoping steps, forming in the second portion of the substrate first andsecond adjacent emitter regions of the first conductivity type near thefirst surface, the first emitter region extending substantially to thefirst surface and laterally separated from the buried layer collectorregion; and during one or more second doping steps, forming in thesecond portion of the substrate first and second adjacent base regionsof the second conductivity type, respectively, beneath the first andsecond emitter regions, wherein the first base region has a first baseregion doping concentration and the second base region has a second baseregion doping concentration different than the first base region dopingconcentration, and the first emitter region and first base regionproviding a first NP or PN junction at a first depth beneath the uppersurface, and the second emitter region and the second base regionproviding a second NP or PN junction at a second depth beneath the uppersurface.
 2. The method of claim 1, further comprising providing at leastone isolation region substantially at the upper surface, wherein thesecond emitter region and the second base region substantially underliethe at least one isolation region.
 3. The method of claim 1, wherein theone or more first doping steps and the one or more second doping stepsare performed during one or more chain implants of dopants of oppositeconductivity type.
 4. The method of claim 1, wherein the buried layercollector region and the second base region are laterally arranged sothat the buried layer collector region extends underneath the secondbase region.
 5. The method of claim 1, wherein the buried layercollector region and the second base region are laterally arranged sothat the buried layer collector region does not extend underneath thesecond base region.
 6. The method of claim 1, further comprising forminga third emitter region and third base region having an NP or PN junctionthere between, laterally arranged on a side of the first emitter regionopposite the second emitter region.
 7. A method for producing a bipolartransistor, comprising: forming first emitter region of a firstconductivity type in a semiconductor substrate having a first emitterregion doping concentration; forming a first base region of a second,opposite conductivity type in the semiconductor substrate underlying thefirst emitter region and having a first base region dopingconcentration, the first base region forming a first PN or NP junctionwith the first emitter region at a first depth from a first surface ofthe semiconductor substrate; forming a second emitter region of thefirst conductivity type in the semiconductor substrate having a secondemitter region doping concentration and Ohmically coupled to the firstemitter region; forming a second base region of the second conductivitytype in the semiconductor substrate underlying the second emitter regionand having a second base region doping concentration greater than thefirst base doping concentration, the second base region forming a secondPN or NP junction with the second emitter region at a second depth fromthe first surface less than the first depth; and forming a buried layercollector region of the first conductivity type in the substrateunderlying the first surface and laterally spaced a third distance fromthe first emitter region.
 8. The method of claim 7, wherein the firstemitter region and the second emitter region are formed at laterallyadjacent locations.
 9. The method of claim 7, wherein the first baseregion and the second base region are formed at laterally adjacentlocations.
 10. The method of claim 7, wherein the second emitter regionis formed to extend laterally at least partly between the first emitterregion and the buried layer collector region.
 11. The method of claim 7,wherein the second base region is formed to extend laterally at leastpartly between the first base region and the buried layer collectorregion.
 12. The method of claim 7, further comprising: forming a thirdemitter region of a third emitter doping concentration coupled to thefirst emitter region; and forming a third base region of a third basedoping concentration coupled to the first base region, the third basedoping concentration is less than the first base doping concentration.13. The method of claim 7, wherein the second emitter region is formedto have a lateral extent greater than the third distance.
 14. The methodof claim 7, wherein the second emitter region is formed to have alateral extent less than the third distance.
 15. A method for producinga bipolar transistor, comprising: forming a first emitter region of afirst conductivity type in a semiconductor substrate having a firstemitter region doping concentration; forming a first base region of asecond, opposite conductivity type in the semiconductor substrateunderlying the first emitter region and having a first base regiondoping concentration, the first base region forming a first PN or NPjunction with the first emitter region at a first depth from a firstsurface of the semiconductor substrate; forming a second emitter regionof the first conductivity type in the semiconductor substrate having asecond emitter region doping concentration and Ohmically coupled to thefirst emitter region; forming a second base region of the secondconductivity type in the semiconductor substrate underlying the secondemitter region and having a second base region doping concentration, thesecond base region forming a second PN or NP junction with the secondemitter region at a second depth from the first surface, the seconddepth shallower than the first depth; forming an isolation featureoverlying the second emitter region; and forming an emitter contact inthe first emitter region and adjacent the isolation feature, thejuncture between the emitter contact and the isolation feature generallyaligning with the juncture between the first emitter region and thesecond emitter region and the juncture between the first base region andthe second base region.
 16. The method of claim 15 wherein the secondbase doping concentration is greater than the first base dopingconcentration.
 17. The method of claim 15, wherein the first emitterregion and the second emitter region are formed in an upper epitaxiallayer of the semiconductor substrate.
 18. The method of claim 17,wherein the buried layer collector region is formed, at least insubstantial part, in a lower epitaxial layer of the semiconductorsubstrate.
 19. The method of claim 15, further comprising: forming abase contact over the semiconductor substrate; forming a WELL region ofthe second conductivity type in ohmic contact with the base contact; andforming a buried layer collector region of the first conductivity typein the semiconductor substrate underlying the first surface and having aportion extending under the WELL region; wherein the second emitterregion is formed to extend laterally from the first emitter region tothe WELL region.
 20. The method of claim 15, wherein the second emitterregion is formed to have a doping concentration less than the firstemitter region doping concentration by a factor of about 1 to 100, andwherein the second base region is formed to have a doping concentrationgreater than the first base region doping concentration by a factor ofabout 1 to 10.